1. Field Of The Invention
This invention is directed to a pressurized water reactor having a safety system grade system for automatically blocking withdrawal of control rods in response to a dropped control rod.
2. Background Of The Invention
The reactivity of a pressurized water reactor is controlled by regulating the concentration of a neutron absorber, such as boron, in reactor coolant circulated through the reactor core, and by control rods which can be inserted into the reactor core. Changes in boron concentration have a core wide effect while the insertion of control rods is more localized. Typically, the control rods are stepped into and out of the core, but can be dropped into the core rapidly to shut down the core should the need arise. It is possible that during normal operation one or more individual control rod drives could malfunction and drop control rods into the core. This results in a reduction in the reactivity of the core with consequent lowering of the average temperature of coolant exiting the core. When this lowering of the average temperature of the coolant exiting the core is detected by the control system, about ten seconds after the actual rod drop, the conventional control system responds to this reduction in temperature by withdrawing specified control rods in order to raise the core average temperature to a set point level. This can result in excessive heat rise in another part of the core as the control system attempts to compensate for the reduction in core reactivity.
In a conventional pressurized water reactor, regulation of the boron concentration is used to control power level with the control rods being manipulated to control power distribution during transients. Even when load following with such a control strategy, only about one-third of the control rods are inserted into the reactor core at power. It has been analytically determined that with such a control scheme, even in the worst case, a dropped rod will not result in a dangerous over-temperature condition in another part of the core. Hence, while a dropped rod has an adverse effect on the operation of a conventional reactor, it is not a critical safety item.
Assignee of the present invention has developed an advanced pressurized water reactor which is protected by passive safety systems. That is, no operator intervention is required to maintain safe operating conditions in the reactor despite various postulated malfunctions. The control strategy for this advanced pressurized water reactor calls for load following primarily with the control rods only and not through regulation of the boron concentration. This results in a wide variation in the combinations of banks of control rods inserted into the core to follow the load and maintain proper power distribution in the core. This makes it impractical to analytically determine whether, with all the possible combinations of rod insertions, there is no situation where a dropped rod would not cause fuel damage in another part of the core.
Thus, there is a need with the advanced pressurized water reactor operated to load follow with the control rods rather than through regulation of boron concentration to have a reliable system for determining if there is a dropped rod. In order to meet the criteria of the advanced pressurized water reactor that all protection systems be passive, any system for detecting a dropped rod must be safety system grade. That is, it must have the degree of reliability that it can operate automatically without the intervention of the human operator. The safety system grade standards are set forth in IEEE Std. 603-1980 which is hereby incorporated by reference. The IEEE Std. 603-1980 standards are mandated by the U.S. Nuclear Regulatory Commission for applications over which the NRC has jurisdiction in Regulatory Guide 1.153 which is also incorporated by reference herein.
It is known to have rod position indicators which track the stepping of the control rods in and out of the reactor core to provide an indication of rod position. It is also known to have rod bottom lights actuated by microswitches when a rod is fully inserted. However, neither of these systems is safety system grade. There are some safety system grade control rod position indicator systems, but they are expensive and cumbersome to maintain.
U.S. Pat. No. 4,774,049 discloses a system which generates on-line, real time displays of reactor core power distributions, and in particular precisely calculates and displays two dimensional core power distributions relative to a reference position. With the use of the described system a skilled human observer can extract an indication of a dropped control rod. However, this system is not of safety system grade and, more importantly, it is not passive. Furthermore, it cannot readily allow the human observer to recognize a failing thermocouple.
There is a need, therefore, for an improved, fully automatic system and method for identifying a dropped rod in a pressurized water reactor, and in particular for such a system which is safety system grade.
There is also a need for such a system and method which can distinguish between a dropped rod and a failure in the system itself.